Well, I did the West Country on a three day weekend once. Stonehenge, Tintagel (castle on crumbling cliff), Dartmoor (half hour to cross wastelands are so cute), possible Moor ponies, hedges, behind which lurk nothing, but one suspects trolls, good stuff.

~Any of the museums. *_* ~Have hot chocolate at Amato's, near the theatre Mary Poppins is playing. Their hot chocolate was my absolute favorite thing to drink in London.~Catch a play. I heard Woman in White is particularly fantastic.~See the Tower Bridge and the Tower of London (in particular to gawk at the Crown Jewels and see the place of death of Anne Bolyn).~ Go shopping! XD

Castles are cool. Oddly enough HP does not really tweak my urge to go visit movie locations. Which is pretty odd, given how many things do. (Now if I could visit Hogwarts, that would be a different story...)

I highly recommend the Imperial War Museum for historical geeking-out in London itself, and a short train and bus ride out into the country gets you to Avebury, which has a stone circle that is not only larger that Stonhenge (there's a village inside it), but is not all touristed, so you can still go up and touch them.